The present invention relates to the field of marine seismic exploration. More particularly, the invention relates to an improved apparatus and method for improving marine cable tow efficiency by improved fairings attached to a marine cable.
Cables are towed through water in marine seismic operations to pull acoustic energy sources, flotation buoys, hydrophones, and other marine seismic equipment through the water. For large seismic vessels, multiple cables are simultaneously towed in a large swath through the water. The cables may extend thousands of meters behind the seismic tow vessel.
Large drag forces are generated as the cables are towed through the water. The drag forces are caused by friction between the water and cable exterior surface and increase exponentially as the vessel speed increases. Uneven water flow around the cables produces alternating shedding forces which cause transverse vibrations, known as xe2x80x9cstrummingxe2x80x9d, in the cables. Strumming creates turbulent flow within the water and generates acoustic noise interfering with seismic data collection operations. Strumming further generates stresses at equipment connection points and accelerates equipment failure.
Numerous techniques have been developed to reduce frictional drag forces acting on a cable towed through water. One technique uses multiple fibers or xe2x80x9chairsxe2x80x9d to interrupt vortex shedding and other friction inducing processes. For example, U.S. Pat. No. 4,084,065 to Swenson (1978) disclosed yarn or fiber incorporated into a braided cable for providing a plurality of polyester fiber hairs for reducing low velocity cable strumming in water. U.S. Pat. No. 4,756,270 to Boscov (1988) disclosed cable fairing comprised of fairing hairs formed with a filamentary yarn woven into a cable.
Another technique uses protrusions extending outwardly from the cable to interrupt drag inducing eddies. U.S. Pat. No. 4,190,012 to Rispin et al. (1980) disclosed a cable fairing having a plurality of stubs formed in a spiral pattern about the exterior surface of a cable. U.S. Pat. No. 5,214,244 to Cummings et al. (1993) disclosed a flexible jacket having a helically displaced phase shifter for decoupling a seismic cable from strumming induced by von Karmen vortex sheets. Each of the phase shifting sections had a length less than one quarter the coherence length of the flexible jacket.
Other friction reducing techniques attach fairings to cables to reduce cable strumming in the water. U.S. Pat. No. 4,075,967 to Silvey (1978) disclosed a hydrodynamic fairing having a plurality of parallel vanes. U.S. Pat. No. 4,470,651 to Hale (1987) disclosed a fairing having a relatively large clearance relative to the cable to permit frictionless rocking and consequential wear between the fairing and the cable. U.S. Pat. No. 4,655,155 to Folb et al. (1987) disclosed a faired towline having a rectangular leading edge. U.S. Pat. No. 5,275,120 to Ruffa et al. (1994) disclosed a tow cable fairing having a plurality of radially extending tab members indexed in a helical pattern longitudinally along the cable.
Various fairing configurations have been developed. U.S. Pat. No. 5,335,620 to Small (1993) disclosed a continuous, longitudinally extending fairing comprised of a single plastic extrusion and fastening means for providing shock and impact protection to seismic array sensors and cables. U.S. Pat. No. 5,367,971 to Carpenter et al. (1994) disclosed an outer cable layer having a twisted surface to create ridges for minimizing vortex shedding and cable strumming. U.S. Pat. No. 5,678,504 to Toplosky (1997) disclosed a towing cable fitted with fairings having negative lift vanes.
Improvements to fairings and the connection mechanisms between fairings and cables were shown in U.S. Pat. No. 4,542,708 to Holcombe et al. (1985), which disclosed a fairing cable segment having a rotatable spring steel clip for minimizing interfacial friction between the fairing and cable.
As the size of marine seismic arrays and the tow speed increases, a need exists for improved techniques for reducing drag forces. The techniques should reduce strumming noise interfering with acoustic data collection and should increase tow efficiency through the water.
The invention provides an apparatus and method for reducing friction acting against a marine cable towable through water. The apparatus comprises a fairing body engagable with the cable, wherein the body has an exterior surface in contact with the water, and a plurality of indentations in the body exterior surface for reducing friction as the body is towed through the water. In different embodiments of the invention, the indentions can comprise dimples, can be symmetrically shaped, can be oriented in a selected pattern, and can have different sizes and configurations.
The method of the invention comprises the steps of attaching a fairing to the cable, wherein the fairing has an exterior surface having a plurality of indentations, of deploying a cable and attached fairing in the water behind a tow vessel, and of moving the tow vessel to tow said cable through the water.